2 Answers
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No. The F and G are heard as the subdominant and dominant of the home key. When listening you have the sense of being away from the C chord and wanting to return to it. A modulation sets up a new home key. It can be temporary (like a vacation home) or permanent (like moving to a new home, for instance the modulation just before the last chorus of "Grandma Got Run Over By A Reindeer").

You get a vote up for reminding me about them being heard as D and S. I see wikipedia article on modulation refers to Tonicization as a kind of "smaller" modulation, where the original tonic is still present. Is it a tonicization then?
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horshNov 30 '11 at 2:12

I suppose you could consider it a modulation; but it's normally considered part of the "changes".

It's as if C-mixolydian, F-mixolydian and G-mixolydian were all superposed together to create a C-"blues" scale. C-D-Eb-E-F-G-A-Bb-B-C. The notes that belong to the "superscale" but not the "current" scale are the blue notes.

But the "full" blues scale is constructed by using the dorian scales: C-dorian, F-dorian and G-dorian, giving: C-D-Eb-E-F-F#-G-Ab-A-Bb-B-C. This gives you moreblue notes against the mixolydian harmony.

You get a vote up for explaining the merge of scales. Somehow your and Mark's answers together give almost the complete picture. Except I think I need a reference to an explanation of mode progressions in the super-scales as opposed to chord progressions in the scales. Could it be explained in the books on jazz harmony?
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horshNov 30 '11 at 2:34

It would have to be in a book on Jazz harmony; although I've never read one. Historically, I think, Jazz developed out of Blues; like a creole language from a pidgin language.
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luser droogNov 30 '11 at 5:18

Now that you mention it, the dorian construction is probably a Jazz revision; the origin of the blues scale was probably based on minor pentatonic.
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luser droogNov 30 '11 at 5:21